The present invention relates to a strobo control circuit which controls an illumination timing of a strobo unit contained in a still camera with a programming shutter employing shutter blades having the function of the diaphragm, and more particularly to a strobo control circuit suitable for the still camera adapted to be supplied with film speed information in the form of a CAS (abbreviation of Camera Autosensing System) code expressed by an insulated pattern formed on a conductive membrane on a cartridge.
As well-known, since the intensity of illumination by the strobo unit is inversely propotional to the square of the distance to a subject to be photographed and the F number of a photographing lens is a geometric series of 21/2, the guide number GN which is an index representing an amount of illumination of the strobo unit is defined by a product of the distance and the F number in the case of the film speed of ISO 100. When a strobo unit having a fixed guide number GN is employed, the strobo unit is lighted at a time when an F number determined in accordance with the film speed and the distance is obtained so that a proper exposure can be obtained.
Since the programming shutter employing the shutter blades having the function of diaphragm possesses the F number which varies during the exposure with the lapse of time, the following methods are adopted generally in order to obtain a proper exposure in the photographing operation using the strobo unit.
(1) An operating range of an opening mechanism for the shutter blades is limited by a mechanism interlocked with both a film speed dial and a focus adjusting mechanism so that the fully opened position of the shutter blades is mechanically limited to determine an F number upon the flashlight illumination of the strobo unit.
(2) An analog operation is performed on the basis of a voltage interlocked with the film speed dial and a voltage interlocked with the focus adjusting mechanism to calculate a timing that the opening position of the shutter blades corresponds to a proper F number and the strobo unit is lighted at that timing.
In the method described in the above item (1), however, since there is provided with the mechanism for limiting the operating range of the opening mechanism for the shutter blades in addition to the mechanism for opening and closing the shutter, it is pointed out that the shutter mechanism is more complicated and it is very difficult to introduce such a method into a camera which does not contain the film speed dial such as a camera for a CAS film.
If the method described in the above item (2) is applied to the camera supplied with the coded film speed such as the camera for the CAS film, the digital-to-analog conversion is required and it is very difficult to eliminate the conversion error and the operation error since a dynamic range of 2.sup.6 is required in the case where the film speed range of a used film assumes ISO25 to ISO1600, for example,